This invention is related to welding thermoplastic materials by ultrasonic energy and, more specifically, concerns a method for welding thin thermoplastic film by ultrasonic energy using gas or air as a coupling medium between the source of energy and the material to be welded.
Welding of thermoplastic film by ultrasonic energy is well known in the art, see for instance U.S. Pat. No. 2,633,894 issued to P. B. Carwile dated Apr. 7, 1953 entitled "Plastic Welding" and U.S. Pat. No. 3,022,814 issued to A. G. Bodine, Jr. dated Feb. 27, 1962 entitled "Method and Apparatus for Sonic Bonding". In the prior art devices, a resonator rendered resonant at an ultrasonic frequency, also known as horn, tool, mechanical amplitude transformer etc., is brought into forced contact with two superposed thermoplastic films to be sealed. The dissipation of ultrasonic energy in the plastic film material causes heating and welding of the material portion contacted by the resonator. In this manner, spot welds are achieved or when the material underneath the resonator is moved a seam weld is provided as described in the patents supra.
The present invention concerns the welding of thin film material without bringing the resonator into physical contact with the film material. In this manner it has been possible to weld film material which is characterized by extreme thinness and material which is difficult to weld.
Quite specifically, in the present invention a support surface having a cavity is provided and film material to be welded is superposed upon the support surface. Ultrasonic energy produced by a resonator is coupled to the film material via an intermediate column or layer of gas, such as air, at the area of the cavity, whereby a weld is achieved. It is important to note that in the present arrangement the output surface of the resonator is not in physical contact with the thermoplastic film material, but is spaced a small distance therefrom so that the ultrasonic energy is coupled to the thermoplastic film by means of an intermediate layer or column of air.
The present invention is particularly suited for welding extremely thin thermoplastic sheet material. For instance, it has been possible to weld two films, each 0.001 inch thick, to each other whereby each layer comprised non-oriented low density polyethelyne film, a material which normally is welded only with great difficulty.
One of the important objects of this invention, therefore, is the provision of a method for we1ding thin film material by ultrasonic energy.
Another important object of this invention is the provision of a method for welding thin film material by ultrasonic energy without the material to be welded being contacted by the source of ultrasonic energy, such as a resonator.
Another important object of this invention is the provision of a method for welding superposed thin thermoplastic film material using an ultrasonically resonating resonator and a layer of gas, such as air, for coupling the ultrasonic energy to the film material.
Still another important object of this invention is the provision of a method for welding thin thermoplastic film material using a support surface having a cavity and coupling ultrasonic energy to the film material at the area of the cavity by means of a resonator and a layer of gas disposed between the film material and the output surface of the resonator.
Further and still other objects of this invention will be more clearly apparent by reference to the following description when taken in conjunction with the accompanying drawings.